The invention relates to a lithium-aluminosilicate glass ceramic of a particular kind and to a method for preparing such a glass ceramic.
The invention further relates to a composite component comprising at least a first component of a lithium-aluminosilicate glass ceramic and at least a second component of a metal alloy having a low coefficient of thermal expansion.
The invention further relates to novel utilizations of a lithium-aluminosilicate glass ceramic.
Since years the applicant has successfully used Zerodur® glass ceramics as precision parts for optical and mechanical applications. The glass ceramic Zerodur® is a lithium-aluminosilicate glass ceramic that is prepared from a base glass of the system Li2O—Al2O3—SiO2, wherein by the addition of nucleation agents such as TiO2 or ZrO2 a controlled crystallization is effected (confer German examined application DE 1,902,432). In addition, from U.S. Pat. No. 4,851,372 a similar glass ceramic has become known which is marketed by the applicant under the mark Zerodur-M®.
These glass ceramics are prepared in several steps. After melting and hot forming usually the base glass is cooled to a temperature below the glass transition temperature. Thereafter the base glass is transformed into a glass ceramic article by controlled crystallization. This ceramization is performed by an annealing process having several steps in which in the beginning nuclei are formed by nucleation at a temperature between 600 and 800° C., usually from TiO2 or ZrO2/TiO2 mixed crystals. Also SnO2 may take part in the nucleation. During a subsequent raise of temperature high quartz mixed crystals grow on these nuclei at a crystallization temperature of about 750 to 900° C. Herein the volume fraction between the crystalline high quartz mixed crystal phase and the glassy phase can be controlled in such a way that a coefficient of thermal expansion of about 0 is reached. To this end normally a fraction of about 80 vol.-% high quartz mixed crystals to about 20 vol.-% residual glass is desired. By controlling the fraction between the crystalline phase of high quartz mixed crystals and the residual glass the characteristics may be adjusted within particular bounds.
However, the range of application of these glass ceramics is limited to about 600° C., while already between 130° C. and 300° C. particular restrictions exist.
In addition, it is known from U.S. Pat. No. 6,515,263, that glasses of the system Li2O—Al2O3—SiO2 may be transformed into glass ceramics (LAS glass ceramics) having high quartz mixed crystals and/or keatite mixed crystals as dominant crystals phases. If after nucleation in the range between 600° C. and 800° C. a further temperature increase until about 900 to 1200° C. is performed, then the previously formed high quartz mixed crystals further transform into keatite mixed crystals (U.S. Pat. No. 6,515,263). The transformation into keatite mixed crystals comes together with a crystal growth, i.e. an increase in crystal size, this leading to an increased light dispersion, i.e. light transmission is reduced at the same time. The glass ceramic article thereby has an increasingly opaque appearance. According to U.S. Pat. No. 6,515,263 a short-time temperature increase up to 1100° C. or more is performed, whereby the glass ceramic is transformed into a ceramic having predominantly a keatite mixed crystal phase in the core and having a high quartz mixed crystal phase close to the surface. These glass ceramics have a coefficient of thermal expansion smaller than 1.5×10−6/K.
However, formed bodies prepared from such a glass ceramic up to now are merely utilized as components in series production. These formed bodies are utilized in transparent or opaque state as cooking surfaces or cooking utensils or as fire-proof glass, as fireplace glasses, cooking utensils or as windows for pyrolysis hearths. In particular, for the application as cooking surfaces, a coefficient of thermal expansion smaller than 1.5×10−6/K is necessary, and a coefficient of thermal expansion of 0×10−6/K is, preferably, desired during manufacture.
In addition, for particular applications composite components are desired which consist of a first component of a glass ceramic and of a second component of a metal. For instance, the glass ceramic component may have the necessary precision, shape precision and temperature stability, while the second component of metal is necessary to guarantee a precise mounting of the composite component and a stable connection technique.
In the manufacture of composite components of a ceramic or glass ceramic and of a metal the major problem normally rests in the differences between the coefficients of thermal expansion, since metals have a tendency to a coefficient of thermal expansion that is considerably higher than that of ceramics or glass ceramics.